The unseen universe

by Balfour Stewart; Peter Guthrie Tait

(b.) When work is done against molecular forces, we have a similar storing up, as, for instance, in drawing a bow or in winding up a watch.

(c.) When work is done against the inertia of a body, i.e. to accelerate its velocity, Newton’s definitions show that the additional kinetic energy so produced is equal to the work so spent.

(d.) In abstract dynamics we simply consider as lost the work spent against friction. In Newton’s time it was not known what became of it.

101. Leaving out, then, for the present, the fourth alternative, we see that whatever work is spent, we must, according to Newton, even in abstract dynamics recognise that it is not lost, but only transformed into an equivalent quantity stored up for future use, either in a quiescent form (as, for instance, the potential energy of a raised weight or bent spring), or in an active form (as the kinetic energy of a moving mass). Here, then, at last, we recognise the same sort of conservation as that which we found in matter. But the statement so far is defective, as we have seen, in one particular. What becomes of work spent in overcoming friction? or what becomes of the energy of the blacksmith’s hammer after it has struck the anvil? To this experiment alone can give the answer. Let us see what it has told us.

Man has been called a reasoning animal, a laughing animal, etc., according to the momentary whim or humour of the classifier; but he is perhaps still more definitely separated from all other animals when specified as the ‘cooking animal.’ Now, it has always appeared to us as something little short of marvellous that, even for the high purpose of cooking his food, or of inflicting exquisite torture on a vanquished foe, savage man should ever have hit upon the process of procuring fire by friction. Considering his condition, and comparing his opportunities and his success with those of even our greatest modern physicists, we cannot but look upon this as one of the very greatest and most notable discoveries ever made in physics. All the more notable, too, from the fact that a man like Newton, though of course aware of it, absolutely missed its significance even at the very moment when it alone was wanted to fill a serious lacuna in one of his grandest and most important practical generalisations.

The missing link was all but supplied by Rumford and Davy at the very end of last century. Rumford’s boiling of water by the heat generated in the boring of a cannon, and Davy’s melting of ice by friction in vacuo, were each conclusively demonstrative alike of the non-materiality of heat and of the ultimate fate of work spent in friction, which is thus seen to be converted into heat; or at least these experiments could easily have been made demonstrative by very slight additions to, or modifications of, their author’s methods or reasoning. But the exact and formal enuntiation of the equivalence of heat and work required to fill the lacuna in Newton’s statement was first given by Davy in 1812.

102. Let us here pause for a moment and contemplate the position to which the solution of our problem had even then attained. Visible kinetic energy, such as that of a cannon-ball shot upwards, is transformed as it rises into visible potential energy. As the ball descends its energy is retransformed from the potential into the kinetic variety until, when it is about to strike the earth, it has, or rather would have if there were no atmosphere, as much kinetic energy as it had when it was first shot upwards.

When the ball has once struck the earth its kinetic energy of visible motion is changed by impact into that kinetic energy of invisible motion of its particles which is called heat; and, generally speaking, in all cases of friction, percussion, and atmospheric resistance we have a change of visible energy into heat, as for instance when a railway train is stopped by the action of the brake, when a blacksmith strikes the anvil with his hammer, when a cannon-ball moves through and heats the air, or when a meteorite or falling-star is rendered incandescent by the resistance it meets with even in the higher and rarer strata of the atmosphere.

We had thus come to the stage of regarding heat as a species of molecular energy into which visible energy is often transformed, and very soon afterwards it came to be perceived that there were other forms of molecular energy besides heat—some of these being potential and some kinetic. Thus two substances may possess mutual chemical affinity when separated from each other, just as a raised stone tends to fall again to the earth, and we obtain a form of potential energy in the one case as truly as in the other. When, for instance, we have carbon or coal in our cellars or our mines, and oxygen in the air, we are in possession of a store of chemical potential energy upon which we can draw at any moment and change it during the process of combustion from the potential to the kinetic form. Again, in a current of electricity we have no doubt a species of kinetic energy, although it still puzzles men of science to say what form of invisible motion such a current implies. From all this, without being further perplexed with scientific details, our readers will perceive that there are many different forms, some of them potential, and others of them kinetic, in which energy may appear.

While we were thus grasping the fact that energy can appear under various forms, we were also beginning to perceive that it had great powers of transmutation—going about from one form to another, and Sir W. R. Grove did good work at this stage of the inquiry in bringing together the various cases of such transmutations in his work on the Correlation of the Physical Forces.